Fuel economies and the need to protect the environment are economic and societal priorities. As a result, it has become desirable to produce elastomers with good mechanical properties so that they can be used in the form of rubber compositions usable for the construction of tires with improved properties, having in particular reduced rolling resistance.
To this end, numerous solutions have been proposed, such as, for example, the use of coupling, starring or functionalizing agents with reinforcing filler to modify elastomers with the goal of obtaining a good interaction between the modified elastomer and the reinforcing filler. In order to obtain the optimum reinforcement properties imparted by a filler, the filler is preferably present in the elastomeric matrix in a final form which is both as finely divided as possible and distributed as homogeneously as possible.
Without being bound by theory, filler particles tend to attract to one another and agglomerate within the elastomeric matrix. As such, there is a reduction in the number of filler-elastomer bonds created during the mixing process. As a result of these interactions the consistency of the rubber composition increases and makes processing more difficult.
Rubber compositions reinforced with fillers, such as, aluminas or aluminum (oxide-)hydroxides, of high dispersibility, and sulfur-vulcanizable diene rubber composition, reinforced with a special precipitated silica of the highly dispersible type, are know in the art. Use of these fillers makes it possible to obtain tires or treads with improved rolling resistance, without adversely affecting the other properties, in particular those of grip, endurance and wear resistance. Although the use of these specific, highly reinforcing, siliceous or aluminous fillers has reduced the difficulties of processing the rubber compositions that contain them, such rubber compositions are nevertheless more difficult to process than rubber compositions filled conventionally with carbon black.
In particular, it is necessary to use a coupling agent, also known as a bonding agent, the function of which is to provide the connection between the surface of the filler particles and the elastomer, while facilitating the dispersion of this filler within the elastomeric matrix.
Sulfur-containing coupling agents used for mineral-filled elastomers involve silanes in which two alkoxysilylalkyl groups are bound, each to one end of a chain of sulfur atoms. The two alkoxysilyl groups are bonded to the chain of sulfur atoms by two similar, and in most cases, identical, hydrocarbon fragments. The general silane structures just described, hereinafter referred to as “simple bis polysulfide silanes,” usually contain a chain of three methylene groups as the two mediating hydrocarbon units. In some cases, the methylene chain is shorter, containing only one or two methylenes per chain. The use of these compounds is primarily as coupling agents for mineral-filled elastomers These coupling agents function by chemically bonding silica or other mineral fillers to polymer when used in rubber applications. Without being bound by theory, it is believed that coupling is accomplished by chemical bond formation between the silane sulfur and the polymer and by hydrolysis of the alkoxysilyl groups and subsequent condensation with silica hydroxyl groups. It is further believed that the reaction of the silane sulfur with the polymer occurs when the S—S bonds are broken and the resulting fragment adds to the polymer. It is further believed that a single linkage to the polymer occurs for each silyl group bonded to the silica. This linkage contains a single, relatively weak C—S and/or S—S bond(s) that forms the weak link between the polymer and the silica. Under high stress, this single C—S and/or S—S linkages may break and therefore contribute to wear of the filled elastomer.
The use of polysulfide silanes coupling agents in the preparation of rubber is well known. These silanes contain two silicon atoms, each of which is bound to a disubstituted hydrocarbon group, and three other groups of which at least one is removable from silicon by hydrolysis. Two such hydrocarbon groups, each with their bound silyl group, are further bound to each end of a chain of at least two sulfur atoms. These structures thus contain two silicon atoms and a single, continuous chain of sulfur atoms of variable length.
Hydrocarbon core polysulfide silanes that feature a central molecular core isolated from the silicon in the molecule by sulfur-sulfur bonds are known in the art. Polysulfide silanes containing a core that is an aminoalkyl group separated from the silicon atom by a single sulfur and a polysulfide group and where the polysulfide group is bonded to the core at a secondary carbon atom are also known in the art. As well as core fragments in which only two polysulfide groups are attached to the core.
However, polysulfide groups that are attached directly to an aromatic core have reduced reactivity with the polymer (rubber). The aromatic core is sterically bulky which may inhibit the reaction. Compositions in which the polysulfides are attached directly to cyclic aliphatic fragments derived by vinyl cyclohexene contain more than one silated core and form large rings. The cyclohexyl core is sterically more hindered than the aromatic core and is less reactive. Although these compositions may be able to form more than one sulfur linkage to the polymer rubber for each attachment of the coupling agent to the silica through the silyl group, their effectiveness is low probably due to the low reactivity.
Without being bound by theory, the low reactivity is due to the attachment of the polysulfide to the secondary carbon of cyclic core structure. The positioning of the polysulfide group is not optimal for reaction with the accelerators and reaction with the polymer.
The present invention overcomes the deficiencies of the aforementioned compositions involving silane coupling agents in several ways. The silanes of the present invention described herein are not limited to two silyl groups nor to one chain of sulfur atoms. In fact, the molecular architecture of the present invention includes multiple polysulfide chains which are oriented in a noncollinear configuration (i.e., branched, in the sense that the branch points occur within the carbon backbone interconnecting the polysulfide chains), and provide a novel configuration.
The fillers of the present invention have advantages over that in the prior art by providing multiple points of sulfur attachment to polymer per point of silicon attachment to filler. The silanes of the fillers described herein may be asymmetric with regard to the groups on the two ends of the sulfur chains. The silyl groups, rather than occurring at the ends of the molecule, tend to occur more centrally and are chemically bonded to the core through carbon-carbon, carbon-sulfur and carbon-silicon bonds. The carbon-sulfur bonds of the thio ester linkages (sulfide) are more stable than the sulfur-sulfur bonds of the disulfide or polysulfide functional groups. These thio ether groups are therefore less likely to react with the accelerators and curing agents or to decompose when subjected to high shear or temperatures normally associated with the mixing and curing of rubber compounds. The thio ether linkage also provide a convenient synthetic route for making the silanes of the present invention. The cyclic core also contains multiple polysulfide groups that are attached to ring by a divalent, straight chain alkylene group. The attachment of the polysulfide group to the primary carbon of the alkylene group decreases significantly the steric hindrance of the core, and increases the reactivity of the polysulfides with the polymer. In addition, the cyclic core orients these alkylene chains containing the polysulfide groups away from each other to further reduce the steric hindrance near the polysulfide groups. This distinction is what allows silane silicon to become and remain bonded (through the intermediacy of a sequence of covalent chemical bonds) to polymer at multiple points using the silanes of the present invention.
Also, without being bound by theory, silated core silanes of the present invention include a Y-core structure. This Y-core structure is believed to enable bonding the polymer at two different points or crosslinking on two different polymer chains, and also enables attachment, such as by bonding, to a filler.